1. Field of the Invention
The present invention relates generally to a shallow trench isolation (STI) structure of a semiconductor device and method for forming an improved shallow trench isolation structure incorporating a pure silicon dioxide STI fill with an oxynitride top layer.
2. Description of the Related Art
Shallow Trench Isolation (STI) is used to electrically isolate semiconductor devices that make up an integrated circuit (IC). STI minimizes unwanted current leakage between the semiconductor devices, which can affect the performance of each semiconductor device. Generally, STI structures are formed by etching trenches into a silicon substrate. These trenches are then filled with an insulating dielectric STI fill such as, silicon dioxide (SiO2). The STI structures are formed early in the semiconductor-integrated circuit fabrication process flow. Therefore, the STI fill is exposed to many additional fabrication processes. These additional fabrication processes include many exposures to hydrofluoric acid (HF). The HF is used to etch away unwanted material, such as silicon dioxide, from exposed surfaces. However, in addition to etching away unwanted material, the HF also erodes away silicon dioxide STI fill. The erosion of the STI silicon dioxide fill occurs in two ways. First, the HF can decrease the overall height of the STI silicon dioxide fill. Second, the HF erodes more STI silicon dioxide fill from around the edges of the top surface of the STI fill. This increased erosion around the edges of the top surface of the STI fill results in the formation of a “divot” between the edges of the top surface of the STI fill and the top surface of the silicon trench walls. The reduction in the overall height of the STI fill and the formation of a divot around the edges of the top surface of the STI fill reduces the effectiveness of the STI structure and can result in a variety of possible circuit failures and/or changes in performance.
One method of minimizing the loss (i.e., erosion) of the STI silicon dioxide fill is to reduce the amount of HF exposure to the minimum amount necessary to achieve a robust integrated circuit fabrication process. However, if HF exposure is already minimized and the erosion still occurs, changes may be made to the STI structure. For example, the composition of the STI fill may be changed to include a dielectric other than pure silicon dioxide in order to reduce the etch rate of the HF. Oxynitride films (e.g., SiOxNy) etch more slowly in HF than SiO2, so methods to create oxynitride features in an STI structure can reduce STI fill height loss and divot formation. Nitrogen ion implantation into the STI silicon dioxide fill is one method of creating an oxynitride (see U.S. Pat. No. 5,316,965 issued to Philipossian et al. on May 31, 1994 and incorporated herein by reference). However, because of the relatively high energy and dose of this implant, the resulting implant damage can actually result in an increased etching rate. Thermal Nitridation of the STI silicon dioxide fill is another method of including an oxynitride in the STI structure (see U.S. Pat. No. 5,811,347 issued to Gardner et al. on Sep. 22, 1998 and incorporated herein by reference). Thermal nitridation is accomplished by exposing a semiconductor wafer to a nitrogen containing gas such as, ammonia (NH3), nitric oxide (NO), or nitrous oxide (N2O), at high temperatures. However, this type of nitridation results in nitrogen incorporation primarily at the silicon trench wall and STI silicon dioxide fill interfaces to form an oxynitride trench liner, which does not protect the top portion of the STI fill from erosion. Another method of including an oxynitride in the STI structure is by directly depositing an oxynitride film into the STI trenches (see U.S. Pat. No. 6,498,383 issued to Beyer et al. on Dec. 24, 2002 and incorporated herein by reference). However, since most common STI fabrication processes are optimized for conformal deposition and chemical mechanical polishing (CMP) of pure silicon dioxide, directly depositing an oxynitride in the STI structure can be problematic in terms of the fill properties and polishing characteristics. Specifically, depositing an oxynitride into the STI trench, as opposed to pure silicon dioxide, may interfere with the chemical mechanical polishing (CMP) process used to remove excess STI fill oxide and to planarize the STI fill (e.g., such that it is approximately level with the pad nitride). The invention described below addresses these issues by providing an STI structure, and a method for forming the STI structure, in which a pure silicon dioxide material is used to fill the shallow trench structure in order to facilitate polishing and in which the upper portion of the silicon dioxide fill, and optionally, the upper STI fill margins, where the silicon dioxide STI fill and the upper edges of the silicon trench walls meet, are doped with nitrogen to prevent erosion.